qemu/xendisk: set maximum number of grants to be used

Legacy (non-pvops) gntdev drivers may require this to be done when the
number of grants intended to be used simultaneously exceeds a certain
driver specific default limit.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -536,6 +536,10 @@ static void blk_alloc(struct XenDevice *
     if (xen_mode != XEN_EMULATE) {
         batch_maps = 1;
     }
+    if (xc_gnttab_set_max_grants(xendev->gnttabdev,
+            max_requests * BLKIF_MAX_SEGMENTS_PER_REQUEST + 1) < 0)
+        xen_be_printf(xendev, 0, "xc_gnttab_set_max_grants failed: %s\n",
+                      strerror(errno));
 }
 
 static int blk_init(struct XenDevice *xendev)